Well, Starfury ships will be .x files (directx), IIRC, so one would assume that SEV ships will be the same.
There are plug-ins for many 3d programs to export to that format. If you can't find one for your program of choice there are file converters that can do it for you.
A question for the shipset designers: If you make a new shipset for Starfury or SEV, can't you also release that set for SEIV, since it's just a matter of rendering to a bmp? For that matter, if you've saved the 3d project file for a SEIV shipset can't you use that for the new games? Just curious.
